Fracture load predictions and measurements for highly toughened epoxy adhesive joints

A method to predict the ultimate strength of adhesive joints has been evaluated for the quasi-static loading of a variety of cracked-lap shear (CLS) and single-lap shear (SLS) joints bonded with a high-strength, toughened epoxy adhesive. The adhesive strength was experimentally characterized in terms of the steady-state critical strain energy release rate, Gcs, as a function of the loading phase angle, using double cantilever beam (DCB) joints. Comparing the calculated energy release rate using the J  -integral with the Gcs at the corresponding phase angle, the ultimate failure load in the fracture joint was predicted and compared with experimental results. When the toughening of the adhesive during subcritical crack growth (i.e. its R-curve behavior) was considered in the analysis, good agreement between the predicted and experimental failure loads was achieved, both for joints made with aluminum or steel adherends. The initial condition at the end of joint overlap (fillet or precrack) did not affect the ultimate joint strength because of the significant amount of subcritical crack growth.